This invention relates to a power control for an externally fired heat engine, and more particularly to a Stirling engine power control.
The Stirling engine is a high efficiency power source which is quiet and reliable. It has an external combustor which can use any heat producing fuel source and can be adjusted for low undesirable combustion emissions. Because the Stirling engine solves so many problems which have remained unsolved in the internal combustion engine, it has been the subject of intense development effort in recent years.
Much improvement has been made in Stirling engine technology and this engine is now approaching the stage at which it will be considered for production development in the numerous applications for which it is ideally adapted. However, before this stage can be reached several problems remain to be solved. These include power control, heat conduction losses, and cost. The power control problem is one which has received much attention and in which great strides have been taken. However, the existing power controls which have been proven and adopted for use are discontinuous in operation or are expensive and complicated and would present a difficult maintenance and service prospect to the average serviceman. In addition to simplicity and low cost, an acceptable practical power control must also provide a fast response time and introduce little or no losses to the engine cycle which would reduce engine efficiency.
Heat conduction loss in the Stirling engine has been a serious problem which has received little attention. The advantage of an isothermal expansion space in the Stirling cycle becomes a liability if a heat conduction path to the cooler permits a significant loss of heat.
The high cost of prior art Stirling engines is partly a consequence of the need to operate the expansion space at very high temperatures. In order to achieve high efficiency, the difference in temperature between the expansion and compression spaces must be as high as possible. However, this requirement imposes severe penalties on the portions of the mechanism which must operate at high temperature such as the piston dome, piston rings, cylinder head, and related seals and connections. The need to operate these components at high temperatures imposes a severe environmental condition on the engine, and the solutions necessary to enable the engine to operate under these conditions are expensive.
Accordingly, a significant advance in the technology of Stirling engines would be achieved by providing a power control apparatus which is simple, reliable, fast acting, and inexpensive, and by incorporating this power control in a system which enables the expansion space to be thermally isolated from the other parts of the engine.